1. Field of the Invention
The invention relates to conditionally-stable operational amplifiers.
2. Description of the Related Art
An operational amplifier is a relatively high gain amplifier capable of being used in various kinds of feedback circuits for performing certain mathematical operations. For example, operational amplifier circuits can provide programmable gain, signal summation, integration, and differentiation, and various other useful functions too numerous to mention here.
The most popular variety of operational amplifier has high-impedance differential signal inputs and a low impedance signal output, and functions as a high-gain differential voltage amplifier. Another kind of operational amplifier, known as an "operational transconductance amplifier," has high-impedance differential signal inputs and a high-impedance signal output, and functions as a differential voltage to current converter.
High accuracy operational amplifier circuits require large gain from zero frequency up to a certain closed-loop bandwidth. Most general-purpose operational amplifiers are constructed with a dominant pole in the open-loop frequency response in order to guarantee stability when any purely resistive voltage divider provides a feedback signal. When the operational amplifier has such an open-loop frequency response, an enormous gain-bandwidth product is required for high accuracy. Therefore, designers of low-power or high-accuracy operational amplifier circuits have considered conditional stability as a way of avoiding the gain-bandwidth product limitation of unconditionally-stable operational amplifiers. A conditionally-stable operational amplifier has at least 180 degrees of phase lag for a frequency less than the frequency at which the operational amplifier has an open-loop unity gain, but the phase lag decreases to less than 180 degrees as the frequency increases to the open-loop unity gain frequency.
One circuit technique proposed for constructing a conditionally-stable operational amplifier is known as multipath conditionally-stable compensation. The multipath technique is introduced in Rudy Eschauzier and Johan H. Huijsing, Frequency Compensation Techniques for Low-Power Operational Amplifiers, Kluwer Academic Publishers, Boston, 1995, pp. 167-173. An operational amplifier using this technique includes a series of integrators, and each integrator includes a first transconductance stage, a second transconductance stage, and a capacitor connected from the input to the output of the second transconductance stage. The series of integrators forms a low-frequency path. Because a series of integrators has at least two integrators and each integrator in practice has slightly more than 90 degrees of phase shift, the series of integrators tends to be neither conditionally stable nor unconditionally stable. In order to make the operational amplifier conditionally stable, the operational amplifier further includes one or more high-frequency bypass paths around the integrators in order to "roll back" the phase shift to less than 180 degrees as the frequency reaches the open-loop unity gain frequency. The bypass path includes a transconductance bypass stage having an input driven by an input of the operational amplifier and an output connected to the second or higher integrator in the series. The output of the transconductance bypass stage is connected to the node interconnecting the first transconductor output, the second transconductance input, and the capacitor of the second or higher integrator in the series. Each integrator stage has a unity-gain frequency (in radians per second) equal to the ratio of the transconductance of its first stage divided by the capacitance of its capacitor. In order for the bypass stage to "roll back" the phase lag, the ratio of the transconductance of the bypass stage to the capacitance of the capacitor to which the bypass stage output is connected must be substantially less than the open-loop unity gain frequency (in radians per second). For a high-order multipath operational amplifier having three integrators, for example, each integrator has the same unity gain frequency, and there are two bypass transconductors, each of which has a take-over frequency (i.e., ratio of the transconductance to the capacitance connected to the bypass transconductor output) that is about one-fifth of the open-loop unity gain frequency.
In practice, a number of related problems have been discovered during the design of a conditionally-stable high-order multi-path operational amplifier. A first problem is the voltage offset and 1/f noise in the low-frequency path. For a high-accuracy operational amplifier, it is desired to have a relatively low voltage offset and low 1/f noise.
A second problem is instability caused when the integrators become saturated by large signals or transients. When an integrator saturates, its open-loop frequency response is changed, so it is possible for the operational amplifier to become unstable.
A third problem is the limited range of closed-loop gain over which the conditionally-stable operational amplifier is stable, and the fact that this limited range of stability may prevent an integrated circuit design from being useful as a general-purpose building block for a variety of applications.